Cryogenic fluids, that is, fluids that have boiling points below -150.degree. F. at atmospheric pressure, are used in a variety of applications. Many of these applications require that the cryogen be supplied as a gas at a pressure around 400 psi. For example, high pressure nitrogen and argon gases are required for laser welding while high pressure nitrogen, oxygen and argon gases are required for laser cutting.
The cryogens for such applications are stored as liquids, however, because one volume of liquid produces many volumes of gas (600-900 volumes of gas per volume of liquid) when the liquid is allowed to vaporize (boil) and warm to ambient temperature. To store an equivalent amount of gas would require that the gas be stored at a very high pressure. This would mandate the use of larger and heavier containers for storage and expensive, high maintenance compressors or pumps to create the storage pressure.
As a result, systems that provide high pressure gas to an application from a relatively low pressure liquid have been developed. Such systems must supply the cryogenic liquid to a heat exchanger at the desired transfer pressure (around 400 psi for the above applications). While special pumps or compressors may be used for this purpose, they feature moving parts that wear and thus require repair, replacement and maintenance. Furthermore, pumps or compressors add considerable cost to the production, and thus purchase price, of a transfer system.
An alternative method of creating the desired transfer pressure is to pressurize the tank within which the bulk supply of cryogenic liquid is stored. Such a system is available from MVE, Inc. of New Prague, Minn. This system utilizes a bulk cryogenic storage tank. The pressure within the storage tank is increased by gravity feeding cryogenic liquid to a heat exchanger. The vapor created thereby is returned to the storage tank so that the pressure within the tank is increased. Cryogenic liquid from within the bulk tank is then delivered to the application at the desired transfer pressure to be used as liquid, or is itself vaporized in another heat exchanger if gas is required.
While this type of system works well, cryogenic bulk storage tanks that are able to withstand pressures in the 400 psi range are expensive when compared to their lower pressure counterparts. It is preferable in many instances for a system to utilize a low pressure tank to store a bulk quantity of cryogenic liquid with cryogenic liquid therefrom flowing to a smaller transfer tank that may be pressurized.
Such transfer tanks may be pressurized in several ways. One way is to heat the fluid in the transfer tank directly with electric heaters. While this will increase the pressure within the transfer tank, it will also warm the cryogenic liquid therein to a saturated state. As a result, if the system sits unused for a period of time, heat leakage into the transfer tank will eventually cause a portion of the saturated cryogenic liquid to boil off. This will potentially increase the pressure in the tank to a level above the tank safety valve setting causing a portion of the cryogenic vapor to be vented to the atmosphere. This is undesirable. Venting is both wasteful and may be unsafe or detrimental to the environment.
Another way of pressurizing a transfer tank is to feed the cryogenic liquid therein to a pressure builder heat exchanger. The vapor produced thereby is then returned to the vapor space above the cryogenic liquid to pressurize the tank. Such a system requires a means of transporting the cryogenic liquid within the transfer tank to the heat exchanger. Many systems use gravity to feed the cryogenic liquid in the tanks to the heat exchangers. While this is effective, many existing tanks do not permit bottom liquid withdrawal and cannot accommodate such an arrangement.
Accordingly, an object of the present invention is to provide a device whereby the pressure within a tank may be increased without significantly heating the liquid contained therein.
Another object of the present invention is to provide a device whereby the pressure within a tank may be increased without the use of pumps or compressors.
Still another object of the present invention is to provide a device whereby the pressure within a top withdrawal tank may be increased.